Methods of external corrosion prevention for a storage tank are known which include substantially complete systems of protection, such as one known as the Steel Tank Institute Protection 3 (STIP 3), which includes coating the outside of the tank with fiberglass, positioning anodes around the outside of the tank connected to supply an electrical potential onto the tank, and providing non-conducting bushings at each pipe and pump connection. Anode protection of the outside of a tank does not affect the internal corrosion. Presently, unless a tank is constructed initially with an internal anode protection system or an expensive tank lining, no good internal corrosion protection system is available.
The problem of internal tank corrosion primarily results from the collection of water or moisture inside of the tank, either from contaminated fuel or oil or from condensation from vented air. In fuel or oil, the water is heavier and collects at the bottom of a steel storage tank. The water typically forms an acid when in contact with liquid hydrocarbons for an extended period of time. Complete and total drainage of the water on a regular basis is not a convenient process and is not undertaken with sufficient frequency to avoid the corrosion. Even small amounts of water can result in an acidic solution which begins a corrosion process. The corrosion process is, in effect, an exchange of electrically charged ions between the steel tank and the acidic liquid. The removal of these ions from the steel into the acidic solution is the corrosion process which may lead to tank cracks and leakage.
In order to prevent this type of corrosion, some storage tanks have in the past been constructed initially with anodes made of zinc or magnesium or the like, inside the tank. During construction, the anodes are positioned at the expected lowest point or points in the tank. The anodes are mounted on steel brackets which space the anode slightly above the low point in the tank and provide a completed electrical circuit across the space between the tank and the anode and then through the welded steel bracket back to the anode. Typically, the zinc or magnesium is molded or cast as a block or cylinder onto the horizontal section of an inverted "U" shaped steel bar or bracket.
In the past, some oil storage tanks were formed with a large open top. Methods of and apparatus for protecting the inside of a metal tank against corrosion were devised as disclosed in U.S. Pat. No. 1,512,557, which included placement of anodes in the tank substantially evenly spaced across the bottom and connected with an electrically conductive cable to a portion of the tank preferably above the level of the liquid. Substantially flat plates of zinc or other metal which is electropositive toward iron was used. The metal anode was spaced apart from the bottom of the tank with electrically insulating materials, such as glass, porcelain, bakelite, or the like, either secured directly to the zinc plate or secured to wooden frames supporting the anode plates. A protective coverings of canvas over the top of the plates was also disclosed in U.S. Pat. No. 1,512,557 in order to prevent the deposit of bottom sediments or products of corrosion upon the plates and on the bottom of the tank. The canvas permitted the passage of electrical current to the plate but shielded the top of the plates from sedimentation. This method and apparatus of anode protection was beneficial for open top tanks in which positioning and placement of the flat plate anodes from above was unobstructed. The flat anodes could be appropriately positioned right-side up. The insulative spacers projected only from one side of the plate. The use of this system is not adequate for closed top tanks, such as underground storage tanks. Available access openings are of limited size and do not accommodate bulky flat plate anodes. Positioning of the flat plate anode assemblies, both right-side up and in a desired location, could not be conveniently accomplished through small openings.
Underground storage tanks, such as gasoline tanks and the like, are now commonplace and their use is much more widespread than open mouth tanks of the past. The closure of the top of the tank has reduced the speed with which water accumulates and has reduced concerns regarding sedimentation. However, the problem of internal corrosion continues to exist and there are many existing tanks which were initially constructed without adequate internal anode protection.
Various anodes have been constructed for their use in water tanks, boilers, water heaters and the like, including those which are enclosed during boiler construction as disclosed in U.S. Pat. No. 635,468. Also, various water tank anodes have been designed to be inserted into previously erected tanks or receptacles, in particular those in which a small clearance distance may exist between the tank opening and adjacent structures or other equipment. One such anode is disclosed in U.S. Pat. No. 2,619,455 which provides an anode which is formed with segments on a bendable rod. Another water tank anode design is disclosed in U.S. Pat. No. 2,666,027 which provides a series of anodes interconnected with a spring connector to avoid breakage of the connecting rod due to bending during insertion. The anodes are suspended from the opening in the water. Proximity to the tank bottom is not a factor in water tanks.
Other anodes have been designed for hot water tanks and the like, which are designed to be threadedly received within an opening, such as an outlet port, and rigidly project from the opening into the water tank. Such threaded anodes are disclosed in U.S. Pat. Nos. 3,867,274; 4,773,977 and 4,786,383. These constructions are designed primarily to be inserted through openings at the bottom of the water heater and apparently project a short distance into the tank. Such devices are not adequate for purposes of large fuel and oil storage tanks with top openings, such as those at gasoline pumping stations.
Coating of the interior of the tank with an insulative epoxy or resin or fiberglass or the like, may reduce corrosion and may facilitate effectiveness of an anode. One such process is disclosed in U.S. Pat. No. 2,941,935 for a tanker ship, where access is presumably more convenient than for an underground storage tank. However, it still requires removing the tanker from operation for a period of time while the coating is applied and the anode is affixed.
Certain anode materials, such as magnesium and aluminum, can produce a hot spark upon metal-to-metal contact with steel, such as by dropping from relatively short distances of several feet Anodes with flexible plastic shielding have been used, particularly in connection with oil transporting trucks or oil tanker ships, which are periodically drained, accessed by workmen for inspections and repairs and thereby exposed to hazards from dropping or contact with other falling metal objects, as disclosed in U.S. Pat. No. 4,171,254. Such anodes are designed to be affixed to the bottom or the walls during construction or while the tank is emptied for inspection or repair. They are not easily adapted for retro-fitting, as by insertion into existing underground fuel storage tanks while they are in operation.
Many underground or closed tanks now in operation have been constructed without internal anodes. To date, there has not been an adequate or convenient way to insert anodes into existing tanks without going through a complicated process which involves taking the tanks out of service. Previously, the tanks had to be drained, cleaned and de-gassed. Then, an opening had to be formed in them of sufficient size for a man to climb inside to either weld anodes supported from the tank at the appropriate bottom locations or alternatively, to completely coat the inside of the tank with a fiberglass or other non-corroding polymeric material. Complete and thorough coating of the interior of the tank has been a difficult process which essentially requires forming an internal fiberglass tank within the existing steel tank. This has been an expensive process and unless properly done to insure complete and total coverage could, in fact, exacerbate the problem. Where portions of the tank were coated and other portions were exposed, a focal point for corrosive activity was formed. In either event, the man-size opening had to be re-sealed as by welding before putting the tank back into service.